1. Field of the Invention
The present invention relates to an electroluminescent (EL) display, and more particularly, to a data driving unit and a scan driving unit that supply a data pulse and a scan pulse to a pixel cell of a panel.
2. Description of the Background Art
Recently, various flat type display devices are being developed to reduce a weight and a volume of a cathode ray tube. The panel display devices include a field emission display (FED), a plasma display panel (PDP) and an electroluminescent (EL) display.
The EL display utilizes an EL phenomenon that a light is generated by a voltage applied to a phosphor layer. Thanks to its rapid response speed, low DC drive voltage and capability of being ultra-thin compared to such an LCD, the EL display can be adaptable to wall-mounting type products or portable products.
The EL displays are classified into an inorganic EL display and an organic EL display depending on its material and structure.
FIG. 1 is a drawing illustrating the cell structure of the inorganic EL panel in accordance with a conventional art.
As shown in FIG. 1, the cell 100 of the inorganic EL panel includes: an upper insulation layer 4 and a lower insulation layer 2, a phosphor layer 3 formed between the lower and upper insulation layers 2 and 4, a back electrode 1 formed on the lower insulation layer 2, and a clear electrode 5 formed on the upper insulation layer 4. The clear electrode 5 is formed at a rear surface of a glass substrate 6.
The upper and lower insulation layers 2 and 4 are made of a dielectric material. Thus, when a voltage is applied to the cell 100, the upper and lower insulation layers 2 and 4 have a certain capacitance.
The phosphor layer 3 is excited by electrons to emit a visible light. The phosphor layer 3 is made of an inorganic substance such as Zns or Mn.
The back electrode 1 is made of a conductive material such as Al. The back electrode 1 receives a scan pulse from a gate driving unit (not shown). That is, the back electrode 1 is used as a scan electrode for supplying the scan pulse to the cells 10.
The clear electrode 5 is made of a clear conductive material such as Indium-Tin-Oxide (ITO). The clear electrode 5 receives a data pulse from a data driving unit (not shown). That is, the clear electrode 5 is used as a data electrode for supplying the data pulse to the cells.
When the scan pulse is supplied to the back electrode 1 and the data pulse is applied to the clear electrode 5 (that is, a voltage is applied between the back electrode 1 and the clear electrode 5), holes are accelerated toward the back electrode 1 and electrons are accelerated toward the clear electrode 5. The electrons and the hole collide at the central portion of the phosphor layer 3. When the electrons and the hole collide, the phosphor layer 3 generates a visible light to display a certain image.
FIG. 2 is a block diagram showing a driving apparatus of the EL panel in accordance with a conventional art.
As shown in FIG. 2, a driving apparatus of an EL display device in accordance with the conventional art includes: pixel cells 100 positioned at cross points between data lines D1˜Dn and scan lines (S1˜Sm) formed on a panel 10; a data driving unit 12 for supplying a data pulse to the data lines D1˜Dn; and a scan driving unit 14 for supplying a scan pulse to the scan lines (S1˜Sm).
The operation of the driving apparatus of the EL display device in accordance with the conventional art will now be described.
First, the data driving unit 12 supplies a data pulse to the data lines D1˜Dn. The scan driving unit 14 supplies a scan pulse and a reset pulse to the scan lines S1˜Sm.
The pixel cell 100 performs an ON/OFF operation by an electric field between the scan electrode receiving a negative (−) scan pulse from the scan driving unit 14 and the data electrode receiving a positive (+) data pulse from the data driving unit 12.
Each pixel cell 100 is equivalently connected to a capacitor (not shown).
The positive (+) pulse is supplied to the data lines D1˜Dn and the negative (−) pulse is supplied to the scan lines S1˜Sm. In this respect, in order to remove the electric charge charged in the pixel cell 100 due to the negative scan pulse, a positive pulse is supplied to the second scan line S1 after the last scan line Sm.
That is, the scan driving unit 14 receives two power sources in order to output a positive (+) pulse and a negative (−) pulse.
FIG. 3 is a circuit diagram showing the scan driving unit of FIG. 2
As shown in FIG. 3, the scan driving unit 14 of the EL display device includes: a scan pulse supply unit 20 for generating a scan pulse and a scan drive IC (Integrated Circuit) 22 for supplying the scan pulse (SP) supplied from the scan pulse driving unit 20 to one scan line (S1) of the scan lines S1˜Sm.
The scan pulse supply unit 20 includes: first and second switching devices (SW1 and SW2) installed in parallel between a ground voltage (GND) and a scan drive IC 22; a third switching device (SW3) installed between scan pulse voltage source (−Vs) and the scan driver IC 22; and a fourth switching device (SW4) installed between a reset pulse voltage source (Vr) and a scan drive IC 22.
The operation of the scan driving unit will now be described.
The second to fourth switching devices SW1˜SW4 are turned on/off in response to a control signal supplied from a timing controller (not shown).
The first switching device SW1 and the third switching device SW3 supply a scan pulse SP to corresponding scan lines S1˜Sm alternately in response to the control signal supplied from the timing controller.
The second switching device SW2 and the fourth switching device SW4 supply a reset pulse (RP) to every scan line S1˜Sm in response to the control signal supplied from the timing controller.
The first switching device SW1 increases a voltage of the negative (−) scan pulse (SP) up to a ground voltage (GND), and the third switching device SW3 supplies a negative scan pulse (SP).
The second switching device SW2 is operated reversely to the fourth switching device SW4 and lowers down the scan pulse (SP) to a negative polarity.
The fourth switching device SW4 supplies a reset pulse RP to every scan line S1˜Sm.
A resistor (R) is a resistance device for reducing a peak current when an instantaneous voltage is applied to the scan drive IC 22.
A driving method of the EL display device of FIG. 2 will now be described.
FIG. 4 shows waveforms for driving the EL display device of FIG. 2.
As shown in FIG. 4, when third switching device SW3 and second switching device SW2 are turned on in a state that first through fourth switching devices SW1˜SW4 are turned off, the negative (−) scan pulse SP is supplied from the scan pulse voltage source (−Vs) to the first scan line S1 through an internal diode of the scan drive IC 22. Synchronized with the negative scan pulse SP, the data pulse DP is supplied to the data electrodes D1˜Dn.
Thereafter, at the same time when the third switching device SW3 is turned off, the first switching device SW1 is turned on. Accordingly, the first scan line S1 is provided with a ground voltage (GND) by the first switching device SW1.
As the first and the third switching devices SW1 and SW3 are alternately turned on/off, the scan pulse is sequentially supplied to every scan line S1˜Sm.
When the scan pulse SP is supplied to every scan line S1˜Sm, the second switching device SW2 is turned off whereas the fourth switching device SW4 is turned on, so that a positive reset pulse RP is supplied from the reset pulse voltage source Vr to every scan line S1˜Sm.
This process is repeatedly performed to sequentially apply the scan pulse SP and the data pulse DP up to the mth scan line Sm to drive the pixel cell 100 and display a picture.
After the picture is displayed, the positive reset pulse RP is applied to the first˜mth scan lines S1˜Sm. When the reset pulse RP is applied to the first˜mth scan lines S1˜Sm, the electric charges charged in the pixel cell 100 are removed.
As stated above, in order for the scan driving unit 14 to supply the negative (−) scan pulse (−Vs) and the positive (+) reset pulse (RP) to the scan lines S1˜Sm, two power sources, that is, the reset pulse voltage source (Vr) and the scan pulse voltage source (−Vs) are required.
In addition, a circuit construction of the scan driving unit 14 requires a high voltage to satisfy both the positive polarity (+) and the negative polarity (−).
As the high voltage is required, a power consumption is increased, and a switching noise is generated in alternately switching two power sources.
FIG. 5 shows electric charges accumulated in the data electrode and the scan electrode of the EL panel in accordance with the conventional art.
As shown in FIG. 5, in the EL display device of the conventional art, a bias voltage of the same polarity is applied between the data electrode and the scan electrode.
That is, negative electric charges are charged in the data electrode, and positive (+) electric charges are charged in the scan electrode, resulting in that uneven luminance phenomenon takes place in the pixel cell or line. This phenomenon causes a cross talk, and thus, an afterimage remains in implementing a fast mobile picture, degrading a picture quality.
In addition, as for the EL display device, since a threshold voltage differs depending on red/green/blue fluorescent materials (R, G, B) in terms of the characteristics of a fluorescent material, if each fluorescent material is omitted by driving the pixel cell with the same voltage, luminance characteristics are degraded.
In this respect, the threshold voltage is a voltage required for emitting light from each fluorescent material, and each fluorescent material has different threshold voltage.
For example, the red (R) and the green (G) fluorescent materials are omitted by a threshold voltage between approximately 150˜240V, and the blue (B) fluorescent material is omitted by a threshold voltage between approximately 120˜200V.
That is, if the fluorescent materials (R, G, B) are omitted with the 240V, since the threshold voltages of each fluorescent material (R, G, B) are different from each other, each fluorescent material has different luminance value.